Snowpack study indicates drier future

By Dan Vergano, USA TODAY

Northern Rocky Mountain snowpack levels over the past five decades are sharply down compared with the past eight centuries, federal researchers reported Thursday.

By Laury Bly, USA TODAY

Youngsters frolic in the snow at Glacier National Park in northwest Montana. Snowpack levels in the Northern Rockies are sharply down in the past 50 years, when compared with the past eight centuries, according to a new study released Thursday.

Youngsters frolic in the snow at Glacier National Park in northwest Montana. Snowpack levels in the Northern Rockies are sharply down in the past 50 years, when compared with the past eight centuries, according to a new study released Thursday.

Despite this year's wet weather replenishing mountain snow, the finding suggests drier times are ahead for the West. In particular, the northern Rockies' snowpack feeds the Columbia, Colorado and Missouri rivers, now the primary water sources for 70 million people.

The historical snowpack reconstruction results, dating to the year 1200 and released by the journal Science, suggest that global warming has broken the normal seesaw pattern of snowpack in the region, in which a down year in the northern Rockies will be offset by a higher snow year in the southern Rockies.

Overall, the average yearly snowpack across the northern Rockies — directly known from snow records to have dropped 30% to 60% in the past 50 years — has fallen more sharply in that time than for any period in the past 800 years, the study shows.

"Temperature is the driver here," says study lead author Greg Pederson of the U.S. Geological Survey's Northern Rocky Mountain Science Center in Bozeman, Mont. "It doesn't take a rocket scientist to know that if temperatures get warmer, snow and ice melt sooner."

In the study, researchers used 66 collections of federal tree growth-ring records gathered across Western states to reconstruct year-to-year snowpack sizes. Trees grow every year to an extent that depends on water. By comparing growth rings from large numbers of dead and living trees, the study scientists developed a record of how much water was available, an amount driven by snowpack, going as far back as about 1200.

"The most important part is this puts the observations of snowpack declines we've seen for a while into the long-term picture," says atmospheric scientist Philip Mote of Oregon State University in Corvallis, who was not part of the study team.

"We can now say the decline really hasn't happened before and now we have tens of millions of people depending on the snowpack for water," Mote says.

In 2009, the U.S. Global Change Research Program estimated average temperatures have risen 1.5 degrees Fahrenheit in the Northwest, over the past century.

Water melts at 32 degrees, and small shifts in temperature can drive widespread early melting of the snow that yearly refills rivers, Pederson says, leaving less for summer months.

The Northern Rockies stretch from Washington state to Montana. The study records show two-decade-long drops in snowpack across the northern Rockies in the 1300s and 1500s that resemble the decline seen in the 20th century, but those declines lasted for shorter periods of time and came when far fewer people were dependent on the snowpack.

"Water demand, as much as supply, is the problem," Pederson says. "We have a lot of fisheries and hydropower relying on this water as well."

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